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[2003:cb:c70b:2d00:73d7:5fab:cc8a:e48c]) by smtp.gmail.com with ESMTPSA id m188-20020a1c26c5000000b0039c4945c753sm6198629wmm.39.2022.06.16.08.04.17 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 16 Jun 2022 08:04:18 -0700 (PDT) Message-ID: <20f49e70-32e0-a141-907c-5f58c543d70b@redhat.com> Date: Thu, 16 Jun 2022 17:04:17 +0200 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.9.0 Subject: Re: [RESEND PATCH] mm: page_alloc: validate buddy before check the migratetype To: Zi Yan , Xianting Tian Cc: Guo Ren , Greg Kroah-Hartman , Andrew Morton , Vlastimil Babka , Linux-MM , Linux Kernel Mailing List , stable@vger.kernel.org, huanyi.xj@alibaba-inc.com, zjb194813@alibaba-inc.com, tianhu.hh@alibaba-inc.com, Hanjun Guo , Joonsoo Kim , Laura Abbott References: <20220613131046.3009889-1-xianting.tian@linux.alibaba.com> <0262A4FB-5A9B-47D3-8F1A-995509F56279@nvidia.com> <435B45C3-E6A5-43B2-A5A2-318C748691FC@nvidia.com> <18330D9A-F433-4136-A226-F24173293BF3@nvidia.com> <5526fab6-c7e1-bddc-912b-e4d9b2769d4e@linux.alibaba.com> <417EC421-DC05-4B35-954B-35DF873A2C40@nvidia.com> From: David Hildenbrand Organization: Red Hat In-Reply-To: <417EC421-DC05-4B35-954B-35DF873A2C40@nvidia.com> X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit ARC-Authentication-Results: i=1; imf30.hostedemail.com; dkim=pass header.d=redhat.com header.s=mimecast20190719 header.b=Zqf5HtQm; dmarc=pass (policy=none) header.from=redhat.com; spf=none (imf30.hostedemail.com: domain of david@redhat.com has no SPF policy when checking 170.10.133.124) smtp.mailfrom=david@redhat.com ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1655391864; h=from:from:sender:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references:dkim-signature; bh=1yesHcFM1zTB7NLF9xER3YapmNtnj5s5fkPYetaCeZg=; b=R8gC/RAgxVeRdHnULmp9YUrGHOwjs7Lgyc0bevQ2ofp58YSOGBdMcB73AcFv1WiRJ3E4ql afXM3Kl8Hx6uAFOZVw7wQ34fOb8H8ZdeDelCexAB1gW2XHRP9wm/n5lqgxJd2/8PEPv2Mi WUhbYBMXomnzgW6xGQuL6//8YFi6I6I= ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1655391864; a=rsa-sha256; cv=none; b=WDE8vYvxNV4RcqurdJrVR0cgVHIIycB8LL3ZFPXBD0Lg35C3U2mRJbudIBqg85djG+uNKH rmi8wjG4m+MPNAmToWeZULArFDqRQtI9taMO1FLswecnb8ethXX+r13iTjWohQC/XYOSGs eWv6WjXndOCfBRMaguLIy/4eLHJ1LAY= X-Rspam-User: X-Stat-Signature: ara4qgyzaro4c9wexyaiotwmy4o69pqa X-Rspamd-Queue-Id: 1F09E80011 Authentication-Results: imf30.hostedemail.com; dkim=pass header.d=redhat.com header.s=mimecast20190719 header.b=Zqf5HtQm; dmarc=pass (policy=none) header.from=redhat.com; spf=none (imf30.hostedemail.com: domain of david@redhat.com has no SPF policy when checking 170.10.133.124) smtp.mailfrom=david@redhat.com X-Rspamd-Server: rspam05 X-HE-Tag: 1655391863-45879 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On 16.06.22 16:01, Zi Yan wrote: > On 15 Jun 2022, at 12:15, Xianting Tian wrote: > >> 在 2022/6/15 下午9:55, Zi Yan 写道: >>> On 15 Jun 2022, at 2:47, Xianting Tian wrote: >>> >>>> 在 2022/6/14 上午8:14, Zi Yan 写道: >>>>> On 13 Jun 2022, at 19:47, Guo Ren wrote: >>>>> >>>>>> On Tue, Jun 14, 2022 at 3:49 AM Zi Yan wrote: >>>>>>> On 13 Jun 2022, at 12:32, Guo Ren wrote: >>>>>>> >>>>>>>> On Mon, Jun 13, 2022 at 11:23 PM Zi Yan wrote: >>>>>>>>> Hi Xianting, >>>>>>>>> >>>>>>>>> Thanks for your patch. >>>>>>>>> >>>>>>>>> On 13 Jun 2022, at 9:10, Xianting Tian wrote: >>>>>>>>> >>>>>>>>>> Commit 787af64d05cd ("mm: page_alloc: validate buddy before check its migratetype.") >>>>>>>>>> added buddy check code. But unfortunately, this fix isn't backported to >>>>>>>>>> linux-5.17.y and the former stable branches. The reason is it added wrong >>>>>>>>>> fixes message: >>>>>>>>>> Fixes: 1dd214b8f21c ("mm: page_alloc: avoid merging non-fallbackable >>>>>>>>>> pageblocks with others") >>>>>>>>> No, the Fixes tag is right. The commit above does need to validate buddy. >>>>>>>> I think Xianting is right. The “Fixes:" tag is not accurate and the >>>>>>>> page_is_buddy() is necessary here. >>>>>>>> >>>>>>>> This patch could be applied to the early version of the stable tree >>>>>>>> (eg: Linux-5.10.y, not the master tree) >>>>>>> This is quite misleading. Commit 787af64d05cd applies does not mean it is >>>>>>> intended to fix the preexisting bug. Also it does not apply cleanly >>>>>>> to commit d9dddbf55667, there is a clear indentation mismatch. At best, >>>>>>> you can say the way of 787af64d05cd fixing 1dd214b8f21c also fixes d9dddbf55667. >>>>>>> There is no way you can apply 787af64d05cd to earlier trees and call it a day. >>>>>>> >>>>>>> You can mention 787af64d05cd that it fixes a bug in 1dd214b8f21c and there is >>>>>>> a similar bug in d9dddbf55667 that can be fixed in a similar way too. Saying >>>>>>> the fixes message is wrong just misleads people, making them think there is >>>>>>> no bug in 1dd214b8f21c. We need to be clear about this. >>>>>> First, d9dddbf55667 is earlier than 1dd214b8f21c in Linus tree. The >>>>>> origin fixes could cover the Linux-5.0.y tree if they give the >>>>>> accurate commit number and that is the cause we want to point out. >>>>> Yes, I got that d9dddbf55667 is earlier and commit 787af64d05cd fixes >>>>> the issue introduced by d9dddbf55667. But my point is that 787af64d05cd >>>>> is not intended to fix d9dddbf55667 and saying it has a wrong fixes >>>>> message is misleading. This is the point I want to make. >>>>> >>>>>> Second, if the patch is for d9dddbf55667 then it could cover any tree >>>>>> in the stable repo. Actually, we only know Linux-5.10.y has the >>>>>> problem. >>>>> But it is not and does not apply to d9dddbf55667 cleanly. >>>>> >>>>>> Maybe, Gregkh could help to direct us on how to deal with the issue: >>>>>> (Fixup a bug which only belongs to the former stable branch.) >>>>>> >>>>> I think you just need to send this patch without saying “commit >>>>> 787af64d05cd fixes message is wrong” would be a good start. You also >>>>> need extra fix to mm/page_isolation.c for kernels between 5.15 and 5.17 >>>>> (inclusive). So there will need to be two patches: >>>>> >>>>> 1) your patch to stable tree prior to 5.15 and >>>>> >>>>> 2) your patch with an additional mm/page_isolation.c fix to stable tree >>>>> between 5.15 and 5.17. >>>>> >>>>>>> Also, you will need to fix the mm/page_isolation.c code too to make this patch >>>>>>> complete, unless you can show that PFN=0x1000 is never going to be encountered >>>>>>> in the mm/page_isolation.c code I mentioned below. >>>>>> No, we needn't fix mm/page_isolation.c in linux-5.10.y, because it had >>>>>> pfn_valid_within(buddy_pfn) check after __find_buddy_pfn() to prevent >>>>>> buddy_pfn=0. >>>>>> The root cause comes from __find_buddy_pfn(): >>>>>> return page_pfn ^ (1 << order); >>>>> Right. But pfn_valid_within() was removed since 5.15. So your fix is >>>>> required for kernels between 5.15 and 5.17 (inclusive). >>>>> >>>>>> When page_pfn is the same as the order size, it will return the >>>>>> previous buddy not the next. That is the only exception for this >>>>>> algorithm, right? >>>>>> >>>>>> >>>>>> >>>>>> >>>>>> In fact, the bug is a very long time to reproduce and is not easy to >>>>>> debug, so we want to contribute it to the community to prevent other >>>>>> guys from wasting time. Although there is no new patch at all. >>>>> Thanks for your reporting and sending out the patch. I really >>>>> appreciate it. We definitely need your inputs. Throughout the email >>>>> thread, I am trying to help you clarify the bug and how to fix it >>>>> properly: >>>>> >>>>> 1. The commit 787af64d05cd does not apply cleanly to commits >>>>> d9dddbf55667, meaning you cannot just cherry-pick that commit to >>>>> fix the issue. That is why we need your patch to fix the issue. >>>>> And saying it has a wrong fixes message in this patch’s git log is >>>>> misleading. >>>>> >>>>> 2. For kernels between 5.15 and 5.17 (inclusive), an additional fix >>>>> to mm/page_isolation.c is also needed, since pfn_valid_within() was >>>>> removed since 5.15 and the issue can appear during page isolation. >>>>> >>>>> 3. For kernels before 5.15, this patch will apply. >>>> Zi Yan, Guo Ren, >>>> >>>> I think we still need some imporvemnt for MASTER branch, as we discussed above, we will get an illegal buddy page if buddy_pfn is 0, >>>> >>>> within page_is_buddy(), it still use the illegal buddy page to do the check. I think in most of cases, page_is_buddy() can return false,  but it still may return true with very low probablity. >>> Can you elaborate more on this? What kind of page can lead to page_is_buddy() >>> returning true? You said it is buddy_pfn is 0, but if the page is reserved, >>> if (!page_is_guard(buddy) && !PageBuddy(buddy)) should return false. >>> Maybe show us the dump_page() that offending page. >>> >>> Thanks. >> >> Let‘s take the issue we met on RISC-V arch for example, >> >> pfn_base is 512 as we reserved 2M RAM for opensbi, mem_map's value is 0xffffffe07e205000, which is the page address of PFN 512. >> >> __find_buddy_pfn() returned 0 for PFN 0x2000 with order 0xd. >> We know PFN 0 is not a valid pfn for buddy system, because 512 is the first PFN for buddy system. >> >> Then it use below code to get buddy page with buddy_pfn 0: >> buddy = page + (buddy_pfn - pfn); >> So buddy page address is: >> 0xffffffe07e1fe000 = (struct page*)0xffffffe07e26e000 + (0 - 0x2000) >> >> we can know this buddy page's address is less than mem_map(0xffffffe07e1fe000 < 0xffffffe07e205000), >> actually 0xffffffe07e1fe000 is not a valid page's address. If we use 0xffffffe07e1fe000 >> as the page's address to extract the value of a member in 'struct page', we may get an uncertain value. >> That's why I say page_is_buddy() may return true with very low probablity. >> >> So I think we need to add the code the verify buddy_pfn in the first place: >> pfn_valid(buddy_pfn) >> > > +DavidH on how memory section works. > > This 2MB RAM reservation does not sound right to me. How does it work in sparsemem? > RISC-V has SECTION_SIZE_BITS=27, i.e., 128MB a section. All pages within > a section should have their corresponding struct page (mem_map). So in this case, > the first 2MB pages should have mem_map and can be marked as PageReserved. As a > result, page_is_buddy() will return false. Yes. Unless there is a BUG :) init_unavailable_range() is supposed to initialize the memap of unavailable ranges and mark it reserved. I wonder if we're missing a case in memmap_init(), to also initialize holes at the beginning of a section, before RAM (we do handle sections in a special way if the end of RAM falls in the middle of a section). If it's not initialized, it might contain garbage. -- Thanks, David / dhildenb